A common problem associated with the distributing devices is that the fluid to be distributed is introduced to a column through a pipe, which has a relatively small diameter compared to the diameter of the column. The fluid has to be distributed evenly over the entire cross-section area of the column and with minimum time delay and time delay distribution. Likewise a collecting device should collect fluid from a column evenly and with minimum time delay and time delay distribution and convey the fluid further to a pipe. This is especially important and hard to accomplish when the cross-section area of the column is large, and/or especially when the column length is short. Both the distributing and collecting devices should have a minimum mixing volume of the fluid fronts. This means that e.g. in a chromatographic column operation the concentration gradient between for example the feed and the eluent should stay distinctive, and this also enables the feeding of the separation profile into a subsequent column, if needed and when needed. To minimize the mixing volume the devices should be in close connection with the column filling material. At the same time the devices should prohibit the column filling material from clogging into the distributing or collecting device. When the distributing and/or collecting devices are situated completely outside the column filling material they enable geometrically ideal shape for the column filling material bed. The distributing and collecting devices should also have a low pressure drop.
In U.S. Pat. No. 4,537,217 is described a fluid separator apparatus and a method of fluid distribution adapted for chromatography applications. The fluid separator apparatus comprises distribution plates, which have recursive channels on one side of the plate and evenly distributed holes on the other side of the plate. The recursive channels have substantially uniform length and similar geometric flow resistance. Channels with recursive T-joints are also shown in the application. However, there are several disadvantages related to this realisation. One of the embodiments of the invention described in the US patent is applicable as such only in a column, which is square in cross-section. In the US patent is described also a solution for columns with circular cross-section. For circular columns the separator apparatus comprises distribution openings, which are located within areas defined by the perimeters of concentric circles. However, the application for columns with circular cross-section is very difficult to scale up to be used in columns with substantially larger diameter than 0,3 m.
In U.S. Pat. No. 4,604,199 is described a filtration column which has a bottom with reciprocally arranged peaks and troughs. In the troughs lie branched pipes with uniformly distributed small holes on their lower portions and which are surrounded by screens or wedge pipes. The branched pipes lead to gathering pipes, which in turn lead to an outlet. This kind of arrangement is prone to mechanical and constructional problems caused by the expanding and contracting of the column filling material. One problem associated with this kind of arrangement is that the device causes heavy mixing in the flowing fluid fronts and this means that the separation medium is not able to work efficiently. Heavy mixing in the fluid fronts is caused because the device is arranged within the column filling material.
In U.S. Pat. No. 5,423,982 is described a liquid chromatography column adapted for in situ chemical sterilisation. The column includes a distributor for distributing fluid conducted through the fluid distribution channel over the entity of the orifice. The distributor is preferably a metal plate, either in the form of a multilayer sintered metal filter, a perforated plate with a hole diameter less than the lower grain diameter of the resin particles, or a woven and/or sintered stainless steel monolayer welded onto a metal ring.
In U.S. Pat. No. 5,324,426 is described a chromatographic column in which one or more of the end plates defining the column are provided with specially designed lands and grooves to distribute the input liquid over the column cross-section area. The distribution plates comprise radially oriented fluid passages of decreasing depth going from the centre of the plate to the circumference.
In U.S. Pat. No. 5,141,635 is described a fluid distributor, which comprises a separator consisting of a disc of porous material, and a distribution plate which comprises on its face annular channels connected to a feed/discharge line by conduits. These channels are joined by a hole through the plate, which has a pressure drop. The pressure drop is inversely proportional to the area of the channels. The US patent also describes the use of porous plates between the resin bed and the distribution plates preventing the resin from entering the channels.
In U.S. Pat. No. 5,354,460 is described fluid transfer system with uniform fluid distributor. In the distributor step-down nozzles with recursive flow channels are used. The step-down nozzles are arranged side by side in concentric rings around a center well.
In U.S. Pat. No. 4,565,216 is described a device for gravimetric distribution of liquid for mass and heat transfer columns. The device employs a container with pipe outlets, a plurality of individual distributors in the form of manifolds and metering devices between the container and individual distributors for metering the partial flows of liquid to the individual distributors. In the US patent is shown that the cross-section of the column is divided into 6 sectors and a hexagonal centre part.
Kochergin and Kearney (Zuckerindustrie 126 (2001) no. 1, 51–54) describe fractal structures for fluid distribution. Fractal means in this connection recursive generations of divisions of flow into channels, which are substantially similar. Achieving the needed degree of performance means that a large number of generations must be used in the engineered fractals and this makes the systems very complicated and expensive.
Problems related to the above described prior art solutions are poor liquid distribution to the whole cross-section area of the column or poor liquid collection from the whole cross-section area of the column or complicated and expensive construction of the distributing and/or collecting device, especially when large columns are used. Poor distribution or collection of the fluid e.g. in a chromatographic column operation results in mixed fluid fronts, increased time delay and time delay distribution. Fluid front means the concentration gradient between different components in the moving phase, for example the concentration gradient between the feed and the eluent. Time delay in the distributing and/or collecting device is the volume of the device divided by the flow rate of the fluid. Time delay distribution is the spread of the distributing/collecting times. Minimum time delay distribution means that the fluid introduced to a column is distributed from each point in the distributing device essentially at the same time or that the fluid flowing out of a column is collected from each point in the collecting device essentially at the same time. Many of the prior art solutions result in large mixing volumes of the fluid fronts. The mixing of the fluid fronts results in dilution in the column. This results further in that the efficiency of the column filling material is poorer and this means that the separation of the desired components is inadequate or requires a larger volume of the column filling material. The operating costs increase when dilution takes place in the column.